What is my Maximum Possible DL Speed?

Today, I swapped my BTO Hauwei Modem for a BT BCI modem. My Cabinet is Hauwei I believe. The long and the short is a 2Mbps DL increase coupled with a 2Mbps UL increase. My Profile on BTW Performance Test has also jumped up 2Mbps to 47.5Mbps or thereabouts. Is that my best speed? An estimate from a BT sales site suggests I should get about 51Mbps. Looking at PlusNet's estimate, it was tested yesterday and shows my fixed 40Mbps rate as my Maximum.Where can I find what the actual speed would be?